While an aluminum alloy plate is used as a material for weight saving in a structural member of a transport machine such as an automobile body or an airframe, high alloying for high strength tends to contradict formability into the structural member.
For example, 7000-series aluminum alloy or extra super duralumin (Al-5.5% Zn-2.5% Mg alloy) for the structural member contains an increased amount of strength-increasing element such as Zn or Mg as a typical method for high strength. This however reduces ductility and thus reduces the formability into the structural member. Furthermore, such high alloying causes deterioration in corrosion resistance or an increase in strength due to room-temperature aging (age hardening) during storage. This significantly deteriorates the formability into the structural member. In addition, this leads to low production efficiency of an alloy plate in a rolling step or the like.
Such a contradiction between high strength and formability is extremely difficult to be resolved only by a composition, a microstructure, or a manufacturing method of a simple aluminum alloy plate (single alloy plate) such as the 7000-series aluminum alloy plate and the extra super duralumin plate.
An aluminum alloy clad plate (laminate plate), in which two to four aluminum alloy layers (plates) having different compositions or properties are laminated together, has been known as a measure to solve this problem.
A typical example of such an aluminum alloy clad plate includes an aluminum-alloy brazing sheet for a heat exchanger, the aluminum-alloy brazing sheet having a three or four-layered structure in which a sacrificial anode material of 7000-series aluminum alloy and a 4000-series aluminum alloy brazing material are cladded on a 3000-series aluminum alloy core.
In addition, Patent Literature 1 provides an aluminum alloy material for a vehicle fuel tank, which includes a clad material including a core made of a 5000-series aluminum alloy material for high strength and a skin material made of a 7000-series aluminum alloy material for high corrosion resistance.
Patent Literature 2 provides a method of manufacturing a clad plate, in which differences in melting point between aluminum alloys such as 1000-series, 3000-series, 4000-series, 5000-series, 6000-series, and 7000-series are used to laminate at most four aluminum alloy layers together by continuous casting with a twin roll.
Patent Literature 3 suggests that when a plurality of aluminum alloy layers are laminated together, a Cu anti-corrosion layer is provided between such aluminum alloy layers, and Cu in the Cu anti-corrosion layer is diffused into the aluminum alloy layers bonded by high-temperature heat treatment to improve corrosion resistance of the clad plate.